BLDC motors (also known as electronically commutated or “ECM” motors) are becoming more prevalent in industries that typically did not use BLDC motors. For example, the need for increased efficiency in the heating and air conditioning market has led to the use of BLDC motors for powering the blower in heating, ventilation, and/or air conditioning systems (referred to herein as HVAC systems). An HVAC system is one example of an air-movement system. Other example air-movement systems include refrigerators, furnaces, heat pumps, blowers for gas-fired appliances (e.g., a gas water heater), etc.
Generally, BLDC motors are synchronous electric motors powered by direct-current (“DC”) electricity and have electronic commutation, rather than mechanical commutators and brushes. Further, BLDC motors include a rotor having a plurality of magnetic poles (e.g., a plurality of poles produced with permanent magnets) of alternating polarity disposed on a surface of a rotor core, and a stator that receives electrical power and produces a magnetic field in response thereto. The magnetic field of the stator interacts with a magnetic field of the rotor to cause movement of the rotor.
BLDC motors use a means for determining the position of the rotor in order to commutate the motor. One method of commutating the motor is referred to as “sensorless” motor commutation. Sensorless motor commutation is often performed by sensing the back electromotive force (BEMF) produced by the motor. Typically, the BEMF signal produced in the stator windings is not large enough for sensorless motor commutation until the speed of the rotor reaches about ten percent of the rated motor speed. As a result, a means of starting the motor without using the BEMF signal may be necessary.
One method of starting a three-phase motor is described in U.S. Publication No. 2009/0160384, which is incorporated herein by reference. Typically, to start the BLDC motor, a controller aligns the rotor of the motor to a known position and then accelerates the rotor (e.g., by using the method described in U.S. Pat. No. 8,084,970). Once the rotor reaches a sufficient speed, the rotor is allowed to coast for a short time (e.g., 20-200 ms) while the controller synchronizes the rotor to engage a normal running mode. During this startup process the air-movement system can generate ramp up noise. In particular, the power signal provided to the rotor can generate torsional torque ripple that excites system vibration modes and results in an audible noise for a short period during ramp up.
Improved methods for starting brushless electrical machines and BLDC motors (e.g., the method described in U.S. Publication No. 2012/0274249, which is incorporated herein by reference) avoid ramp up noise during the startup of the motor by generating a three-phase alternating current (AC) voltage signal by all phases of the motor. However, this method still allows the rotor to coast while the controller synchronizes the rotor to engage a normal running mode. Some motors, such as small motors with low inertia, do not coast well. Therefore, there is a need for a further improved method for starting brushless electrical machines and BLDC motors, where the rotor does not need to coast before engaging in normal running mode.